A surface emitting laser (VCSEL; vertical cavity surface emitting LASER) is a semiconductor laser which emits a light in a direction perpendicular to a substrate, has characteristics of lower cost and higher performance relative to an edge emitting laser, and ease of fabricating in arrays. Therefore, studies are being conducted on the VCSEL as a light source for optical communications such as optical interconnections, etc., a light source for an optical pickup, and a light source for an image forming apparatus such as a laser printer, etc., and some of them are being practically applied.
Now, an optical system generally having a semiconductor laser element, etc., including a surface emitting laser element has a problem that a change in a light amount occurs due to feedback light, wherein reflected light from a lens or glass returns to an original laser element. The above-described change in the light amount varies from what occurs at high speed on the order of nanoseconds (ns) to a change which occurs on the order of milliseconds (ms). In the related art, it is considered that the surface emitting laser is resistant to the feedback light since a reflectance of a mirror is high. However, as a result of studies, it has been confirmed that the surface emitting laser is not necessarily resistant to feedback light and that, in particular, in case of a surface emitting laser array in which multiple surface emitting lasers are aligned, light emitted from a certain surface emitting laser becomes the feedback light, which feedback light falls on a neighboring surface emitting laser, so that the change in the light amount occurs. Countermeasures for such feedback light are disclosed in Patent documents 1-3, for example.
Patent document 1 discloses a surface emitting laser, wherein a resonator is formed by a lower multilayer film reflecting mirror and an upper multilayer film reflecting mirror and a relaxation oscillation frequency at a bias point in the resonator is set to exceed an optical communication frequency which modulates a laser beam output from the surface emitting laser.
Moreover, Patent document 2 discloses a surface emitting semiconductor laser, including a semiconductor substrate, an active layer formed at an upper part of the semiconductor substrate, an emission face which is formed at an upper part of the active layer and which emits, in a direction perpendicular to the semiconductor substrate, a laser beam generated in the active layer, and an absorption layer which is provided on the emission surface and which partially absorbs the laser beam.
Furthermore, Patent document 3 discloses a surface emitting laser module for optical transmission, wherein at least a surface emitting laser chip and a monitoring photo detector are mounted on a TO header, the surface emitting laser module including a cap which includes a window coated with a film having a transmittance of no more than 40%.
Now, with the surface emitting laser, a laser beam may be taken out also from a direction opposite a direction of emission of the laser beam, so that the emitted light may be caused to fall on a light receiving element such as a photo diode, etc., to measure an output of the edge emitting laser, and control may be performed based thereupon to control a light amount of laser beams emitted from the edge emitting laser.
However, with the edge emitting laser, light may not be taken out from a face opposite the direction of emission of the laser beam from a structural viewpoint, so that some schemes to provide an optical system, etc., for monitoring the light amount, etc., are necessary.
Such schemes include a scheme in which, in a direction of emission of a laser beam of a surface emitting laser, the laser beam is reflected by a transparent member to be a window that is arranged in a tilted manner, and is caused to fall on a photo detector which is a light receiving element provided inside a surface emitting laser module, so that a light amount is monitored.
However, the above-described scheme has two problems. One is that interface reflectance at upper and lower faces of the transparent member to be the window is low on the order of 4-5%, so that, with such a degree of reflectance, current induced by light received at the light receiving element such as the photo diode, etc., is small and ends up getting buried in noise, etc. As a result, S/N ratio is low, so that it is not possible to accurately control the light amount of the surface emitting laser.
Another one is that, with an etalon effect due to reflection of light at the upper and lower faces of the transparent member to be the window, a change in the light amount occurs due to interference, so that the light amount cannot be monitored accurately. For example, Patent document 4 discloses a method of obtaining a monitor beam of a desired reflection intensity by providing a transparent member to be a window that is tilted, with a reflection control layer. However, while this method uses, as the monitor beam, a beam reflected at an interface of the upper face and the lower face at the transparent member to be the window, a change in a light amount occurs due to optical interference, so that it is not possible to detect the monitor beam in an accurate and stable manner. The same applies to Patent document 5.
Moreover, Patent document 10 discloses a scheme in which is provided a surface emitting laser module having a structure such that the etalon effect does not occur. This scheme provides a structure in which a reflective coating face made of metal is formed at a part of a transparent member to be a window, an emitted beam is reflected at the reflective coating face, and the reflected beam is caused to fall on a photo diode which is a light receiving element. With the above-described scheme, it is inferred that, since a reflectance of the emitted beam at the reflective coating face is high on the order of 50-60%, the beams which pass therethrough decrease and an effect of the etalon effect due to interference decreases. However, a region on which the reflective coating face is provided is around a light spot and a sufficient light amount as a monitor beam cannot be secured, so that a light amount cannot be monitored reliably. Moreover, in the region on which the reflective coating face is provided, the reflectance is high, so that the beams which pass therethrough decrease and a beam shape (a beam profile) of the light spot becomes non-uniform and a distribution of the light amount of the light spot of the beam emitted via the transparent member to be the window ends up becoming a distribution of a shape which deviates from a Gaussian distribution.
Thus, an object of the present invention is to provide an optical device which allows obtaining a monitor beam of a desired light amount without influencing a distribution of a light amount within a light spot, and an optical scanning apparatus and an image forming apparatus using the above-described optical device.